1. Field of the Invention
The present invention relates to a slurry for chemical mechanical polishing and more particularly a polishing slurry that is well suited to a chemical mechanical polishing performed in the step of forming a copper-based metal interconnection of a semiconductor device.
2. Description of the Related Art
In the formation of a semiconductor integrated circuit such as an ULSI, for which the progress to attain further miniaturization and more densely spaced arrangement has been currently gathering more speed, copper is a particularly useful material for electrical connection to obtain the interconnection of good performance and high reliability, because of its considerably low electrical resistance and high resistance against the electromigration and the stress migration.
Since it is difficult to work copper into shape by means of dry etching, a copper interconnection is formed by so-called damascene method, for instance, in the following way.
Firstly, a sunken section such as a trench or a connection hole is formed in an insulating film formed on a silicon substrate. Next, after a barrier metal film is formed on the surface inclusive of the inside of this sunken section, a copper film is grown by the plating method so as to fill up this sunken section. Polishing is then carried out by the chemical mechanical polishing (referred to as “CMP” hereinafter) method until the surface of the insulating film other than the sunken section is completely exposed, so that the surface may be planarized. Thereby, the formation of an electrical connection section such as a buried copper interconnection, a via plug or a contact plug, in which copper is laid to fill up the sunken section, with a barrier metal film being interposed therebetween, is accomplished.
When a buried-type electrical connection section is formed, a barrier metal film is generally formed, as described above, for the purpose of preventing the copper-based metal from diffusing into the insulating film and improving the adhesion between the copper-based metal and the substrate (the insulating film). For the barrier metal film against copper-based metal, tantalum-based metal such as Ta or TaN is favourably used.
To conduct the polishing of a copper-based metal film overlying such a barrier metal film, it is necessary to take the differences in polishing rate between the copper-based metal film and the barrier metal film and that and the insulating film into consideration.
Now, from necessity of filling up the sunken section of the insulating film, the copper-based metal film must be formed thickly. In order to polish and remove such a thick copper-based metal film efficiently, a polishing slurry capable to make the rapid polishing of the copper-based metal film is normally utilized. However, when such a polishing slurry is used for the barrier metal film made of a different material, the polishing rate for this film generally becomes rather low. Therefore, if the barrier metal film is made to be polished and removed thoroughly, even the copper-based metal laid within the sunken section may be excessively removed to create dishing. The phenomenon of this sort is particularly notable when the tantalum-based metal is utilized for the barrier metal film.
Accordingly, in order to prevent dishing of this sort from occurring, there has been proposed the two-steps polishing method which comprises the step of a first polishing wherein mainly a thick copper-based metal film for filling is polished and removed, and the step of a second polishing wherein mainly a barrier metal film is polished and removed (for example, in Japanese Patent Application Laid-open No. 189296/2001).
What is required in the step of the first polishing is that the thick metal film for filling is polished and removed efficiently therein, and the presence of the dishing thereafter is well suppressed. Meanwhile, what is required in the step of the second polishing is that, together with some metal for filling that may remain after the step of the first polishing, the barrier metal film is polished efficiently, using the insulating film as a stopper, so that the polished face may be well planarized.
In such a two-steps polishing method, a CMP slurry having rather a strong mechanical effect than a chemical one is generally employed, in the step of the second polishing wherein mainly a barrier metal film is polished and removed, because the hardness of the barrier metal film is higher than that of the copper-based metal film.
However, if a polishing slurry whose mechanical polishing effect is strong enough to attain a satisfactory polishing rate is used, the insulating film may be excessively polished or its polished face may become rough and, thus, a good electrical connection section becomes hard to be formed. On the other hand, if a polishing slurry whose pH is adjusted to be on the acidic side is used to achieve a high polishing rate for the barrier metal film, its chemical polishing effect becomes stronger and the polishing rate for the copper-based metal for filling, higher, which may result in the creation of dishing and poor planarity of the polished face.
As a conventional method of suppressing dishing, there has been known a method wherein, through the use of a polishing slurry containing benzotriazole or its derivative, a protective film is formed on the surface of the copper film to prevent the ionization of copper by the oxidizing agent and thereby chemical dissolution of copper is suppressed. For instance, in Japanese Patent Application Laid-open No. 83780/1996, it is described that dishing of a copper film in the step of a CMP can be prevented by adding benzotriazole or its derivative into a polishing slurry.
Nevertheless, with a conventional polishing slurry which contains a protective-film forming agent such as benzotriazole, its effect of coating the copper film through formation of the protective film alone has a limitation, and its control over the polishing of the copper film is rather restricted. Especially in the step of the second polishing of the afore-mentioned two-steps polishing method, the polishing rate for the copper film must be lowered much further to improve the planarity of the polished face.